(1) Field of the Invention
The present invention relates to a semiconductor device, and in particular, to a bipolar transistor and a radio frequency amplifier circuit equipped with a bipolar transistor.
(2) Description of the Related Art
At present, a GaAs-MESFET (Metal Semiconductor Field Effect Transistor), GaAs-HEMT (High Electron Mobility Transistor), a hetero-junction bipolar transistor (HBT) or the like are used as an amplifier element for a power amplifier for mobile communication. A hetero-junction bipolar transistor (hereinafter to be referred to as “HBT”), in particular, is advantageous over a GaAs-MESFET in the two following points. First, an HBT does not require a negative power supply and thereby enables single positive power supply operation, and second, it is possible to miniaturize the chip size since collector current density can be increased.
It is generally known that the amount of collector current increases as ON voltage of a base-emitter voltage (hereinafter base-emitter voltage is denoted as “Vbe” and ON voltage between a base and an emitter is denoted as “Vf”) decreases when an element temperature rises. Therefore, when concentration of collector current is caused in a radio frequency power amplifier equipped with plural transistors, the amplifier falls into a vicious circle in which the element temperature rises locally due to the increase in power consumption, which further increases locally the collector current in the element. Thus, uneven amount of current between the transistors does not only affect performance and life span of a power amplifier, but also promotes the concentration of current, so that the transistor falls into the thermal runaway state and breaks down in some cases.
As a solution to such problem as described above, a base ballast resistor, which is connected to the base of a bipolar transistor and gives a negative feedback to the base-emitter voltage (Vbe) when the element temperature rises, has conventionally been used. With such negative feedback to Vbe, it is possible to balance out the increase in the amount of collector current due to the rise in temperature, and prevent thermal runaway. The following describes the conventional technology of the radio frequency amplifier circuit to which the conventional base ballast resistor is applied.
FIG. 1A is an equivalent circuit schematic of a conventional radio frequency amplifier circuit. Bipolar transistors 101-1, 101-2 and 101-n (to be represented as “101” hereinafter) are bipolar transistors of n number of cells. A collector voltage is applied to a collector terminal 115 while an emitter terminal is earthed. A direct-current (DC) bias is provided from a DC terminal 148, and a radio frequency (RF) power is inputted from an RF terminal 149. The DC terminal 148 is connected to base electrodes 105-1, 105-2 and 105-n (hereinafter to be represented as “105”) of the bipolar transistor 101 via a resistance 147 as a base ballast resistor. The RF terminal 149 is connected to the base electrode 105 of the bipolar transistor 101 via a condenser 163. The radio frequency power amplified by the bipolar transistor 101 is outputted from the collector terminal 115.
FIG. 1B shows voltage values and current values at the respective terminals shown in FIG. 1A. It is assumed that a current amplification factor (hFE) of the bipolar transistor 101 is 50, the number of cells is defined as n=20, and a resistance value indicated by the resistance 147 is 5 ohms. In the case where a total amount of the collector current is 1 A and the amount of current is evenly distributed to the bipolar transistors 101, a collector current is 50 mA and a base current is 1 mA for each bipolar transistor 101. The total amount of base current is 20 mA and a voltage drop generated in the resistance 147 is 0.1V. Therefore, when 1.3V is applied to the DC terminal 148, 1.2V is applied to the base electrode 105.
In FIGS. 1A and 1B, the case where current concentration occurs in an arbitrary bipolar transistor is taken as an example. For instance, it is assumed that a collector current of 60 mA that is 1.2 times as much as a collector current of another bipolar transistor 101-n flows in the bipolar transistor 101-2. In this case, since a current amplification factor of the bipolar transistor 101-1 is 50, the base current that flows in the resistance 147 increases from 1 mA to 1.2 mA, and the base current that flows in the resistance 147 increases from 20 mA to 20.2 mA. Due to the increase by 0.2 mA in the base current, the negative feedback of Vbe generated in the resistance 147 is 1 mV at the highest. Due to the increase in the amount of current, however, a junction temperature of the bipolar transistor 101-2 rises from the initial temperature of 80 to 90 degrees Celsius. Such temperature increase by 10 degrees Celsius reduces the ON voltage (Vf) of Vbe by 0.017V. Thus, the negative feedback (1 mV) of Vbe generated in the resistance 147 is smaller than the decrease (17 mV) of Vf due to the increase in temperature so that the amount of collector current continues to increase. More precisely, Vf decreases by 17 mV and the negative feedback of Vbe is 1 mV, and as a result, the total decrease of Vf is 16 mV and the amount of the current which flows in the bipolar transistor 101-2 increases by 60% and thus amounts to 80 mA. Indeed, the amount of negative feedback of Vbe to be obtained increases as the resistance value of the resistance 147 is increased, however, such increase in resistance is not appropriate in this case since it increases the voltage drop in the resistance 147 even in normal operation and the necessary voltage applied to the DC terminal 148 becomes large.
As has been described, the problem of the conventional radio frequency amplifier circuit in FIGS. 1A and 1B is that the negative feedback voltage of Vbe to be obtained in the resistance 147 is not sufficient enough to balance out the decrease of Vf due to the increase in the amount of collector current, therefore, thermal runaway in bipolar transistor cannot be prevented in the case where the amount of collector current of an arbitrary bipolar transistor increases by 20%.
FIG. 2 is an equivalent circuit schematic of another conventional radio frequency amplifier circuit (see specification of U.S. Pat. No. 6,828,816).
The difference between the present radio frequency amplifier circuit and the conventional radio frequency amplifier circuit in FIGS. 1A and 1B is that resistances 146-1, 146-2 and 146-n (hereinafter to be represented by 146) as base ballast resistors are each connected between the DC terminal 148 and the respective base electrodes 105. In the case where the number of cells in the bipolar transistor 101 is defined as n=20, in order to set a value of parallel resistance of the resistance 146 connected between the DC terminal 148 and the base electrode 105 to be 5 ohms, a resistance value of the respective resistances 146 needs to be set to 100 ohms.
As shown in FIG. 2, the resistance 146 is set to each bipolar transistor 101 so that it appears that the negative feedback of Vbe can be increased due to the resistance 146, however, it is hard to say that the effects are sufficiently gained. The reason is that the base electrode 105 of the bipolar transistor 101 is connected to a wiring 145 for transmitting radio frequency. Due to this, in the case where the amount of current in an arbitrary bipolar transistor (e.g. 101-2) increases, the base current which increases accordingly is provided not only through the resistance 146-2 but also from other resistance 146-n via the wiring 145. Such phenomenon can be understood from the point that the base electrode 105-2 and another base electrode 105-n come to have the same potential within one circuit.
As is described above, the problem of the conventional radio frequency amplifier circuit shown in FIG. 2 is that the circuit shown in FIG. 2 is substantially the same as the circuit shown in FIGS. 1A and 1B, and the amount of the negative feedback voltage of Vbe to be obtained in the resistance 146 is the same as that obtained with the conventional radio frequency amplifier circuits shown in FIGS. 1A and 1B, and is still not enough to prevent thermal runaway caused in bipolar transistor.
FIG. 3 is an equivalent circuit schematic of another conventional radio frequency amplifier circuit (see specification of U.S. Pat. No. 5,321,279).
The difference between the present radio frequency amplifier circuit and the conventional radio frequency amplifier circuit shown in FIG. 2 is that condensers 150-1, 150-2 and 150-n (hereinafter to be represented as “150”) are parallely connected to the respective resistances 146-1, 146-2 and 146-n. Ideally, radio frequency power is inputted from the base electrode 105 to the bipolar transistor 101 through the condenser 150. In contrast, a direct-current bias is provided from the base electrode 105 to the bipolar transistor 101 through the resistance 146. It is necessary that a relatively large capacity value is set for the condenser 150 so that a loss of radio frequency is reduced. In the case of setting the number of cells in the bipolar transistor 101 as n=20 and a resistance value of the resistance 146 to be 100 ohms, a value of parallel resistance of the resistance 146 is 5 ohms.
According to the present radio frequency amplifier circuit, it is assumed that the collector current of 60 mA, an equivalent of 1.2 times as much as the collector current of another bipolar transistor 101-n flows in the bipolar transistor 101-2, as is the case of the conventional radio frequency amplifier circuit shown in FIGS. 1A and 1B. Since hFE of the bipolar transistor 101-1 is 50, the base current that flows in the resistance 146-2 increases from 1 mA to 1.2 mA. The resistance 146 being 100 ohms, the negative feedback voltage of Vbe to be generated in the resistance 146 is 20 mV. Due to the increase in the amount of current, a junction temperature of the bipolar transistor 101-2 rises from the initial temperature of 80 to 90 degrees Celsius, and the decrease of ON voltage (Vf) of Vbe due to such temperature increase (10 degrees Celsius) becomes 17 mV. In this case, the negative feedback (20 mV) of Vbe generated in the resistance 146 is larger than the decrease (17 mV) of Vf due to the temperature increase, so that the amount of collector current starts to decease. Thus, with the present radio frequency amplifier circuit, it is possible to balance out the increase in the amount of collector current, by the negative feedback of Vbe, and thereby to prevent the occurrence of thermal runaway.
The problem with the conventional radio frequency amplifier circuit shown in FIG. 3 is that the gain decreases. The reason is that a part of the radio frequency power inputted from the RF terminal 149 is consumed as heat when passing through the resistance 146.
FIG. 4 is another equivalent circuit schematic of the conventional radio frequency amplifier circuit (see U.S. Pat. No. 5,629,648).
The difference between the present radio frequency amplifier circuit and the conventional radio frequency amplifier circuit shown in FIG. 3 is that the radio frequency power inputted from the RF terminal 149 is inputted into the base electrode 105 without passing through the resistance 146. Thus, it is possible to avoid decrease of gain.
Nevertheless, the problem of the conventional radio frequency amplifier circuit shown in FIG. 4 is that each bipolar transistor 101 needs to be equipped with a condenser 150 for letting radio frequency power pass, therefore, the layout becomes complex and the cost of radio frequency amplifier circuit increases due to increase in the chip area.
Another problem with the conventional radio frequency amplifier circuit shown in FIG. 4 is that a mixed flow of radio frequency power and direct-current bias at the terminals 152-1, 152-2 and 152-n (hereinafter to be represented as “152”) causes the radio frequency power to easily leak to the direct-current bias terminal 148 and thereby affects a bias circuit (not shown in the diagram) for providing the direct-current bias terminal 148 with bias. In order to solve this, a ground condenser needs to be connected to the DC terminal 148, which causes another problem that the number of components increases.
FIG. 5A is a cross-sectional view showing the structure of the bipolar transistor 101 in the conventional radio frequency amplifier circuit. FIG. 5B is a plane view of the conventional bipolar transistor 101, and FIG. 5A shows a cross-sectional view of such transistor at the dashed line A-A′. However, in FIG. 5A, an emitter wiring 132 is omitted. As shown in FIG. 5A, on a substrate 118 made of GaAs, a collector contact layer 117 made of n+-type GaAs, a collector layer 109 made of n-type GaAs, a base layer 108 bade of p-type GaAs, an emitter layer 111 made of n-type InGaP, and an emitter contact layer 110 made of n-type InGaAs are sequentially formed. An emitter electrode 113 is formed on the emitter contact layer 110, while a collector electrode 112 is formed on the collector contact layer 117 and a base electrode 107 is formed on the base layer 108. The p-type GaAs forming the base layer 108 has an impurity density of 4×1019 cm−3, a thickness of 80 nm, and a sheet resistor of 250 ohms/sq. In FIG. 5B, the emitter electrode 113 is pulled out while connected to the emitter wiring 132, and is connected to the emitter terminal 102. A signal generated by synthesizing DC and RF is provided from the terminal 103 to the base electrode 107. In order to improve radio frequency characteristic, it is necessary to reduce the base-emitter resistance 122 and thereby to make the gap 119 between the base electrode 107 and the emitter layer 111 shorter. Thus, when the gap 119 is becomes bigger, the base-emitter resistance 122 gets larger so that it is possible to increase a negative feedback voltage of Vbe, however, a loss of radio frequency power increases and the radio frequency characteristic is degraded.
Thus, the problem of the conventional bipolar transistor 101 shown in FIGS. 5A and 5B is that it is not possible to increase the negative feedback voltage of Vbe since the base-emitter resistance 122 needs to be reduced in order to improve radio frequency characteristic because the base electrode 107 is provided with the signal generated by synthesizing DC and RF.
FIG. 6 is a cross-sectional showing the structure of another bipolar transistor in the conventional radio frequency amplifier circuit. The difference between the present bipolar transistor and the bipolar transistor 101 shown in FIGS. 5A and 5B is that two mesa-shaped emitter layers 111 and three base electrodes 107 are formed on the base layer 108. A bipolar transistor having plural emitter layers is generally called “multi-finger type bipolar transistor”. It is necessary, even in a multi-finger bipolar transistor, to reduce a base-emitter resistance 122 in order to enhance the radio frequency characteristic, so that gaps 119-1 through 119-4 between the respective base electrodes 107 and the respective emitter layers 111 need to become closer. The base-emitter gaps 119-1 through 119-4 are designed to have the same length.